Supercritical Water Fluidized Bed Reactor Research Articles

Distribution characteristics of salt crystals in a supercritical water fluidized bed reactor with CFD-PBM coupled model

Supercritical water gasification (SCWG) is a promising method for converting organic waste into hydrogen. However, salt deposition problems seriously hinder the development of hydrogen production from high-salt organic waste in the supercritical water fluidized bed reactor (SCWFBR). This work investigates the distribution characteristics of salt crystals in supercritical water (SCW) by using computational fluid dynamics coupled with population balance model (CFD-PBM). The proposed model is proved to be useful in simulating the nucleation and growth processes of sodium chloride in the SCWFBR. Sodium chloride crystals are concentrated near the nozzle, and crystal mean diameter increases with increasing the mass flow rate of sodium chloride solution and decreasing the mass flow rate of SCW. Crystal size in the SCWFBR can be controlled by adjusting the mass flow rates of salt solution and SCW. The obtained results provide a theoretical basis for the SCWG of high-salt organic waste.

Numerical investigation on fluidization characteristics of binary particles in supercritical water fluidized bed reactor under pulsed conditions

The fluidization properties of binary particles in a supercritical water fluidized bed reactor (SCWFBR) are numerically investigated based on coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations. The accuracy and effectiveness of the CFD-DEM model is firstly validated via previously published experimental data. Then, the model is adopted to study the effect of flow velocities and sinusoidal pulse on the mixing of the binary particles. Numerical results show that it is effective to promote mixing of binary particles in SCWFBR by introducing external energy pulses, which cannot be achieved by simply increasing the flow velocities without pulses. Compared to high frequency pulses, low frequency pulses are more effective in promoting mixing. Within the parameters considered here, it is also found that the pulse amplitude is almost positively correlated with the effect on the promotion of particle mixing when it is smaller than the fixed flow velocity. In addition, an efficient and economical method based on Fast Fourier Transform (FFT) on the bed pressure drop is given to determine the optimal pulse frequency, which can prevent the waste of manpower and the physical resources from happening. This study can provide technical guidance to achieve optimal mixing of binary particles in SCWFBR and offer theoretical support for the design and optimization.

Numerical prediction on minimum fluidization velocity of a supercritical water fluidized bed reactor: Effect of particle shape

Supercritical water fluidized bed reactor (SCWFBR) is a novel gasification reactor for hydrogen production. Compared with traditional hydrogen production technique, the SCW-based gasification has outstanding advantages such as high hydrogen production efficiency, no emission of gaseous pollutants, low water consumption and strong material adaptability. In this paper, based on Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations, numerical investigation is conducted on the effect of particle aspect ratio (α) on the minimum fluidization velocity (Umf) of a SCWFBR containing cylindrical biomass particles. The coupled CFD-DEM model is firstly validated against previously published experimental data. Then, the Umf for six types of particle shapes are calculated under different working conditions of temperature ranging from 633 to 693 K, pressure ranging from 25 to 27 MPa and α ranging from 1 to 6. Numerical results show that Umf increases with the temperature, but decreases with the pressure. Moreover, Umf is increasing with α. The underlying mechanisms are revealed by investigating the micro-structure of different shaped particles. It is shown that the increase of Umf with α is caused by the weakened fluid-solid interaction force and enhanced inter-particle locking. And the weakened fluid-solid interaction force is mainly due to the increase of the voidage. Finally, a predictive correlation of Umf for the SCWFBR is proposed considering α which demonstrates strong predictive capability for cylindrical particles.

Numerical investigation on cold flow dynamics of supercritical water fluidized bed reactor with inclined distributor: Design and scale up

Supercritical water fluidized bed reactor (SCWFBR) is a novel concept for the gasification of coal and biomass to produce hydrogen. In this work, to enhance the mixing in the axial direction, an inclined distributor is introduced to optimize the flow dynamics in SCWFBR with partitioned fluid supply. Through numerical simulations based on the two fluid model (TFM), the effects of the inclined distributor structure and operating parameters on the solid distribution and the residence time are evaluated with the optimal values determined. Numerical results show that, area ratio = 2:1, SCW velocity ratio = 3:1, flow ratio = 3.36:1 and inclination angle = 20° are the optimal design in this paper. A predictive correlation of the minimum fluidization velocity for the improved SCWFBR is also proposed based on the numerical data. The average error between the correlation and numerical simulation results is approximately 1.4% which strongly demonstrates its capability. Finally, based on the optimal design, the lab-scale reactor is further scaled up and the studies about two scale-up rules are carried out. Only the cold flow is simulated in this study without considering chemical reaction which would be involved in future work.

Numerical prediction on the minimum fluidization velocity of a supercritical water fluidized bed reactor: Effect of particle size distributions

The minimum fluidization velocity of a supercritical water fluidization bed reactor (SCWFBR) is numerically nvestigated based on coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations. The accuracy of the CFD-DEM model is firstly validated via previously published experimental data. Then, the model is adopted to study the effects of particle size distributions (PSD) on the minimum fluidization velocity in which four types of PSD including Gaussian-type, Mono-type, Flat-type and Binary-type are considered. Numerical results show that the minimum fluidization velocity for the Flat-type PSD is the smallest among the four while the one for the Mono-type PSD is the largest. The minimum fluidization velocities for the Mono-type and Gaussian-type PSD share quite similar values. However, it is conditionally valid when the largest amount of particle size in the Gaussian-type PSD is equal to its mean particle size which is used in the Mono-type PSD. The agreement of the minimum fluidization velocity between Mono-type and Gaussian-type PSD is also influenced by the number of small particles. The mechanism behind these phenomena is revealed by investigating the micro-structure of differently sized particles. Finally, predictive correlations for the minimum fluidization velocity of the SCWFBR are proposed based on the numerical results which demonstrates strong predictive capability for wide PSD.

Numerical investigation on the scale-up rules of a supercritical water fluidized bed reactor using the two-fluid model

The supercritical water fluidized bed reactor (SCWFBR) is a novel hydrogen production technique, so the understanding on its scale-up is limited. In this regard, the trial-and-error procedure is not an option for traditional experimental research because it is costly and high risk. To overcome these problems, numerical simulations were carried out in this study based on the two-fluid model (TFM) to examine the capability of different scale-up rules for the SCWFBR. The numerical model was first validated based on experimental results. Then, four different-sized SCWFBRs were designed, in which numerical simulations for both air–solid and supercritical water (SCW)–solid systems were conducted following different scale-up rules. The distributions of solid volume fraction, solid velocity and pressure in these reactors were fully investigated. Comparisons among the numerical results showed that keeping the Reynolds number, Froude number and dimensional inlet velocity constant is critical for the scale-up of both SCWFBRs and traditional gas–solid fluidized bed reactors (FBRs). Moreover, keeping the particle diameter constant is helpful in obtaining the similarity of the multiphase flow behavior. For the SCWFBR, but not for the traditional gas–solid FBR, a constant density ratio between solid and fluid should be kept during the scale-up. Finally, for the SCW–solid system with more active particle motions, the effect of the interparticle collisions should be considered in the scaling parameters at high Reynolds numbers.

Simulation of solid-fluid interaction in a supercritical water fluidized bed with a cold jet

Supercritical water fluidized bed reactor, where supercritical water serves as a fluidizing agent, is viewed as a promising way in hydrogen production. Water in cold jet experiences great physical fluctuations through injection under supercritical pressure. Severe temperature changes easily result in heterogeneous flow structures when cold jet injects entraining particles into the reactor. This paper develops an EMMS/energy model based on the energy-minimization multi-scale principles where energy balance equations are added. The effects of temperature on drag force due to simultaneous change of water density and viscosity are investigated. Validation is done with this new drag model incorporated into the Euler-Euler Two-Fluid model. Results show that temperature increase improves heterogeneous flow structure, especially in particles-dilute flow. EMMS/energy model can capture particle acceleration by thermal expansion of water in cold jet, which is responsible for clusters of particles.

Fluidization in Supercritical Water: Heat Transfer between Particle and Supercritical Water

Abstract Supercritical water fluidized bed reactor, which is used to gasify biomass and produce hydrogen, is a new member of fluidized bed family. Forced convection heat transfer between supercritical water and particles is a major basic heat transfer mechanism in supercritical water fluidized bed reactor. The object of this paper was to determine the heat transfer characteristics for a forced convection between a spherical particle and supercritical water (SCW) in a range of pressure from 23 to 27 MPa and temperature from 637 to 697 K. A numerical model fully accounting for thermal physical property variation of SCW has been solved using a finite volume method with Reynolds number up to 200. Comparing with constant property flow, high velocity and temperature gradient in the vicinity of the particle surface were observed when the variable thermal physical property of SCW was incorporated in calculation. Based on the numerical results, a correlation that takes into account the large thermal physical property variation was proposed for predicting Nusselt number.

Numerical Investigation on the Two Phase Flow Behaviors in Supercritical Water Fluidized Bed with Swirling Flow Distributor

ABSTRACTSupercritical water fluidized bed reactor is a promising in the clean and efficient conversion of coal, and the distributor is one of the key component for the heat and mass transfer enhancement. However, the optimization study for the distributor in supercritical water fluidized bed reactor has been seldom conducted due to the special thermal properties of supercritical water. In this work, the swirling flow distributor was designed for its optimization for heat and mass transfer inside a supercritical water fluidized bed reactor. The swirling flow can be generated by the concentric circle or triangle type hole distribution in distributor with 0 or 45° intersection angle between the fluid inlet velocity direction and the distributor plane. The computational particle fluid dynamics (SCWFB) method, which has quick calculating speed and high accuracy, was used in this work to study the particle-fluid two-phase flow behaviors inside SCWFB with swirling flow distributors. Investigations were made to reveal the influence of the hole distribution type and intersection angle on the bed pressure drop and particle volume fraction characteristics. The results showed that the triangle type distributor with 45° intersection angle has the best fluidization performance. The conclusions drawn may has potential application for continuous and stable operation of supercritical water fluidized bed reactor for coal gasification.

Hydrogen production by biomass gasification in a supercritical water fluidized bed reactor: A CFD-DEM study

In this study, we used CFD-DEM model to simulate glucose gasification in a supercritical water fluidized bed reactor (SCWFBR). Simplified reaction kinetic model, including glucose decomposition, water gas shift reaction and methanation reaction, was used. Particle distributions, gas composition and reaction rate profiles were analyzed. Moreover, the influence of chemical reaction on flow behavior, and the effects of operation parameters, such as wall temperature, flow rates and initial bed height were investigated. The results show that high wall temperature, low flow rate and high initial bed height are in favor of gasification. However, lower H2 yield, gasification efficiency (GE) are found at flow rate below the minimum fluidization velocity. The simulation results show good agreement with the experimental results, demonstrating the validity of this model. The information obtained by the numerical study is useful for the operation and optimization of the gasification reactor.

Experimental study on hydrogen production by lignite gasification in supercritical water fluidized bed reactor using external recycle of liquid residual

The technology of supercritical water gasification provides a novel way to convert high-moisture lignite to hydrogen-rich products directly. According to the energy analysis of the whole system for supercritical water gasification, the energy recovery of liquid residual played a key role in the energy efficiency improvement. Moreover, previous research findings showed that some chemical compounds such as phenols and formic acid contained in liquid residual can be recycled to increase the yield of hydrogen. Therefore, an external recycle of liquid residual was used in the supercritical water fluidized bed system to recover not only the heat energy but also the chemical energy by re-gasification of the liquid residual. Besides, the separation of gaseous products from the system enhanced the gasification process. Hydrogen production experiments were operated in this system and the influence of operating temperature 470–550°C, pressure 23–27MPa, concentration 3–35wt% and cycling flow rate 0–70g/min were studied. The experimental results showed that hydrogen yield and carbon gasification efficiency reached 32mol/kg and 82%, respectively, at 530°C.

Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor

Semicoke powders with particle size less than 6 mm are by-products during the pyrolysis of coal. Direct combustion of semicoke powders is difficult due to the low volatile content. Supercritical water gasification might provide an efficient conversion method for semicoke powders. In order to determine the optimum conditions of gasification of semicoke with the supercritical water fluidized bed reactor, the influences of the main operating parameters including temperature (540–660 °C), feedstock concentration (10–30 wt%), flow rate of preheated water (40–80 g/min) and alkali catalysts (K2CO3, KOH, Na2CO3 and NaOH) were systematically investigated in this study. The results showed that semicoke-water slurry of 30 wt% was continuously transported into the reactor and stably gasified without plugging problems. Hydrogen yield of 85.90 mol/kg was obtained with the hydrogen molar fraction of 61.02%. In particular, carbon gasification efficiency of more than 95% was obtained under the conditions of 660 °C, 60 g/min flow rate of preheated water and 10 wt% feedstock concentration with 5 wt% K2CO3.

Industrialization prospects for hydrogen production by coal gasification in supercritical water and novel thermodynamic cycle power generation system with no pollution emission

Energy conversion and utilization, particularly carbon-based fuel burning in air phase, have caused great environmental pollution and serious problems to society. The reactions in water phase may have the potential to realize clean and efficient energy conversion and utilization. Coal gasification in supercritical water is a typical carbon-based fuel conversion process in water phase, and it takes the advantages of the unique chemical and physical properties of supercritical water to convert organic matter in coal to H2 and CO2. N, S, P, Hg and other elements are deposited as inorganic salts to avoid pollution emission. The State Key Laboratory of Multiphase Flow in Power Engineering has obtained extensive experimental and theoretical results based on coal gasification in supercritical water. Supercritical water fluidized bed reactor was developed for coal gasification and seven kinds of typical feedstock were selected. The hydrogen yield covers from 0.67 to 1.74 Nm3/kg and the carbon gasification efficiency is no less than 97%. This technology has a bright future in industrialization not only in electricity generation but also in hydrogen production and high value-added chemicals. Given the gas yield obtained in laboratory-scale unit, the hydrogen production cost is U.S.$ 0.111 Nm3 when the throughput capacity is 2000 t/d. A novel thermodynamic cycle power generation system based on coal gasification in supercritical water was proposed with the obvious advantages of high coal-electricity conversion efficiency and zero pollutant emission. The cost of U.S.$ 3.69 billion for desulfuration, denitration and dust removal in China in 2013 would have been saved with this technology. Five kinds of heat supply methods are analyzed and the rates of return of investment are roughly estimated. An integrated cooperative innovation center called a new type of high-efficient coal gasification technology and its large-scale utilization was founded to enhance the industrialization of the technology vigorously.

Hydrogen production by Zhundong coal gasification in supercritical water

The high sodium content of Zhundong coal restricts its application in direct combustion and supercritical water gasification is a promising clean and efficient coal conversion technology. To obtain the gasification and sodium distribution characteristics of Zhundong coal in supercritical water, experimental investigations were conducted in autoclaves and supercritical water fluidized bed reactors. The experimental results showed that as the concentration of Zhundong coal slurry decreased or the residence time increased, more sodium was released from the solid phase in autoclaves. Carbon gasification efficiency of more than 95% was obtained with the coal slurry of 30 wt%∼40 wt% in the fluidized bed reactor. Less than 0.07% sodium was released from the reactor, in other words, most of the sodium remaining could be collected and caused no pollution and fouling problems for downstream process. Supercritical water fluidized bed reactor appeared to be a promising reactor for supercritical water gasification of Zhundong coal.

Numerical study on biomass model compound gasification in a supercritical water fluidized bed reactor

Supercritical water fluidized bed gasifier is a new promising reactor for thermochemical conversion of wet biomass. However, there exists many troubles in directly measuring the process details in the reactor due to the extreme operating condition. A comprehensive 3-D numerical model of hydrodynamics based on the two-fluid model accompanying with heat transfer and chemical reactions kinetics is developed in this study to simulate glucose gasification in a supercritical water fluidized bed reactor in a temperature range 823–923K under pressure 25MPa. The particle collision is described by the kinetic theory of granular flow. The gasification reaction rates are determined by Arrhenius equation. The flow behavior of supercritical water-particle flow, temperature and reaction rates distribution, gas composition profiles in the bed were predicted and assessed against available published experimental data. The particles show a homogeneous fluidization in supercritical water fluidized bed. Results show that the current three dimensional gasification model can reasonably describe the reacting flow behavior in the supercritical water fluidized bed reactor, which may provide a convenient and low-cost way for the reactor design and optimization.

Numerical study on laminar free convection heat transfer between sphere particle and high pressure water in pseudo-critical zone

Supercritical water fluidized bed reactor (SCWFBR) is a promising new reaction vessel which can effectively gasify wet biomass and efficiently produce hydrogen. Free convection heat transfer from particle in supercritical water (SCW) is a major basic heat transfer mechanism in a fixed bed or fluidized bed with low superficial velocity. In this paper, numerical study on the steady free convection heat transfer around single sphere particle in high pressure water of pseudo-critical zone was carried out. Both the Boussinesq approximation method and real properties model (considering variable specific heat, density, viscosity, and conductivity of SCW) were incorporated to simulate the flow and temperature field. With respect to Boussinesq approximation, real properties model shows higher vorticity and temperature gradients in the vicinity of the sphere surface, which shows variation of thermo-physical property has remarkable effect on the free convection heat transfer process. High local Nusselt number and high heat transfer rate were observed with real properties model.

Hydrogen production by supercritical water gasification of biomass: Particle and residence time distribution in fluidized bed reactor

Particle distribution and residence time distribution (RTD) in supercritical water fluidized bed reactor (SCWFBR) greatly affect the hydrogen yield through determining the two phase mixing and reaction time. A Eulerian model incorporating the kinetic theory for solid particles was applied to simulate the solid distribution and RTD of feeding materials. The effect of four types of feeding methods and feeding rates on solid distribution and RTD were evaluated based on the simulation results. Results showed that double symmetrical feeding pipe with an feeding mouth angle of 45° provides more uniform solid distribution and longer residence time compared with those of other three types. A nonlinear relationship between feeding rate and RTD was observed, and an optimum feeding rate was found to be related to the best solid-fluid mixing in the study.

Numerical Study on the Mixed Convection Heat Transfer between a Sphere Particle and High Pressure Water in Pseudocritical Zone

Mixed convection heat transfer between supercritical water and particles is a major basic problem in supercritical water fluidized bed reactor, but little work focused on this new area in the past. In this paper, a numerical model fully accounting for thermophysical property variation has been established to investigate heat transfer between supercritical water and a single spherical particle under gravity. Flow field, temperature field and Nusselt number are analyzed based on the simulation results. Results show that buoyancy force has a remarkable effect on flow and heat transfer process. When the direction of gravity and flow are opposite, the gravity enhances the heat transfer before the separation point and inhibits the heat transfer after the separation point. When gravity is incorporated in calculation, a higher temperature gradient and a thinner boundary layer in the vicinity of the particle surface are observed before separation point, and the situations are just the reverse after separation point. Variation of specific heat and conductivity plays a main role in determination of heat transfer coefficient.